In a free electron laser with a long, linear wiggler magnet, the external focusing required to keep the electron beam particles from transversely dispersing can seriously degrade laser performance. The transverse focusing modulates the longitudinal velocity of each beam electron, periodically and non-adiabatically changing the phase of each electron with respect to the electromagnetic wave. Phase changes of order unity over a betatron period can strongly detrap or debunch electrons and greatly reduce the gain of a linear wiggler amplifier.
Relevant background work on free electron lasers (FELs), beginning with the initial work of H. Motz, Journal of Applied Physics, 22 527(1950), and continuing through approximately 1981, is discussed in some detail in U.S. Pat. No. 4,500,843, issued to A. Szoke and D. Prosnitz and assigned to the assignee hereof; and this discussion is incorporated herein by reference.
Approximately 32 U.S. Patents directed to FELs have issued since 1971; most of these have issued since 1980. Only one of these patents, U.S. Pat. No. 4,283,687 issued to Elias, Madey and Smith for a FEL with end-tapered wiggler magnet strength, contemplates the use of a helical periodic magnet. The use of such a magnet allows the associated magnetic field to decrease monotonically to zero as one approaches the exit of the magnet region. Helical magnets are quite difficult to construct, and Elias et al do not discuss any alternative designs that would allow transverse focusing that approximates the focusing available with a helical magnet.